With the recent increase in the amount of communication information, rapid progress is being made in various communication systems utilizing the microwave region, such as, e.g., car telephones, satellite communication, and satellite broadcasting, and many microwave dielectric materials have been developed therewith.
Among such microwave dielectric materials, the dielectric porcelains for use in resonators, filters, and the like are required to have the following properties (i) to (iii):
(i) to have a high relative permittivity .epsilon..sub.r ;
(ii) to have a high unloaded quality coefficient Q.sub.u (i.e., a small dielectric loss 1/Q.sub.u); and
(iii) to have a small absolute value of the temperature coefficient .tau..sub.f of resonance frequency f.sub.0 (i.e., a small temperature dependence of f.sub.0).
Dielectric materials such as Ba(Mg.sub.1/3 Ta.sub.2/3)O.sub.3 and Ba(Zn.sub.1/3 Ta.sub.2/3)O.sub.3 have been known to have a small dielectric loss among those properties, while dielectric materials such as BaO-RE.sub.2 O.sub.3 -TiO.sub.2 (wherein RE represents a rare earth element) have been known to have a high relative permittivity.
In JP-A-6-275126 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") is disclosed a dielectric porcelain composition which comprises barium (Ba), bismuth (Bi), titanium (Ti), and oxygen (O) as essential components and may contain neodymium (Nd) and samarium (Sm) as optional components. In JP-A-6-309926 and JP-A-6-325620 is disclosed a dielectric porcelain composition which has been improved in dielectric characteristics by incorporating sodium (Na), potassium (K), and lithium (Li) into the dielectric porcelain composition described above. Specifically, there is a description therein which contains "the value of Q can be improved without changing relative permittivity .epsilon..sub.r ".